Preheat the broiler and position a rack 6 inches from the heat. Using a sharp knife, peel the oranges, removing all of the bitter white pith. Cut each orange crosswise into six 1/3-inch-thick slices. Arrange the orange slices in 4 gratin dishes, overlapping them slightly, and spread with the marmalade. Sprinkle the brown sugar on top and broil for about 10 minutes, shifting the dishes for even browning, until bubbling and lightly caramelized in spots. Transfer to a rack and let cool for 10 minutes.
Meanwhile, using an electric mixer, beat the crème fraîche until lightly whipped. Fold in the crystallized ginger. Spoon the ginger crème fraîche over the orange slices, sprinkle with the shredded mint and serve immediately.